Calibration for optical filter

ABSTRACT

Calibrating each of a plurality of driven optical filters. The color parameters of the driven optical filters are characterized for the individual optical filter. These color parameters are used as calibration data to calibrate more standard information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.60/181,525, filed Feb. 10, 2000.

BACKGROUND

Different kinds of optical filters are known. Optical filters can beformed by coating a blank to form an optical filter which has differentcharacteristics in different locations of the filter.

Examples of such filters are found in U.S. Pat. No. 5,426,576. In theseoptical filters, the amount of color saturation may vary based on thedistance along a gradient axis. The gradient axis can be a lineargradient axis, or a circumferential gradient axis, in this patent. Also,two filters can be used together to form a cross fader.

Different kinds of coated optical filters are also known. In general,these coated optical filters may have characteristics that vary based onthe amount of the coating.

SUMMARY

The present application teaches a system which enables consistent colorfrom each of a plurality of luminaires, each of which use a coatedoptical filter.

According to the present application, a special technique of calibrationis described for an optical filter which has characteristics that varybased on a parameter of the optical filter, e.g. color that changesalong a gradient axis.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail, with referenceto the accompanying drawings, wherein:

FIG. 1 shows a block diagram of the luminaire system;

FIG. 2 shows a flow diagram of forming the filters;

FIG. 3 shows a diagram of rotation compensation for the filtercharacterization.

DETAILED DESCRIPTION

FIG. 1 shows a system in which a light transmission device 90 includinga light assembly 9 with a lamp 10 and reflector 11 is caused to transmitlight along an optical axis 12. A filter assembly 13 has two colorfilters 14 and 15 are placed along the optical axis. Each of the colorfilters, such as 14, may include an alignment mark 125. The alignmentmark 125 may include, for example, a physical hole in the filter. Thealignment mark allows the system to determine a set point in the filter,for example the beginning of a certain color spectrum. The filters arecoated with a varying amount of filtering medium, so that differentareas on the filter produce a different color effect.

The two color filters can be moved relative to one another by the motorsSM1 and SM2. By moving the color filters, the degree of saturationchanges, and hence the color output changes. The motors SM1 and SM2 arecontrolled by a controller 100. The controller 100 operates according toa prestored program which may be stored in its memory 110.

According to the present system, each luminaire 99 should, on cue,produce the same color. This is done according to the present system bycalibrating each of the filters 14, 15 in each luminaire 99 based on areference standard. The calibration allows each luminaire to know anexact position of the filter that produces a specific color effect evenif there are positional and/or color differences between the filters.

The system used herein may use a parametric color filter of the typedescribed in U.S. Pat. No. 5,426,576. Alternatively, any other kind offilter that has characteristics that vary according to a parameter ofthe filter, here a distance along a gradient axis, may be compensatedusing this system.

No two filters, in general, will be exactly the same. One technique usedherein is to provide tight tolerances on certain aspects of the filter,and allow other aspects of the filter to be corrected by the calibrationprocess. For example, spectral form, radiality, start and end points maybe tightly controlled. Other parameters such as relationship of theaperture hole to the coating edge, and linearity, may be more looselycontrolled.

In addition to manufacturing differences in the filter itself, anothersource of errors in the filters may involve the combining of the filters14, 15 with the hub that carries the filters shown as 121. In thisembodiment, the hub includes a magnet 123 which is used to set a ‘home’position of the filter relative to the optical axis 12 of the luminaire99. In assembling the filters, a jig may align the center of the magnet123 to the aperture hole in the filter. Errors in this assembly mayarise, for example due to the physics of the way the manufacturing jigmounts the devices. The jig can not have too tight a fit in the aperturehole, because of the fragile nature of certain filters. For example,certain filters may be manufactured from Vycor(™); a form of fusedsilica manufactured by Corning, Inc. Other sources of manufacture andassembly errors may be also be expected. In general, this may provide anerror up to plus or minus 0.5 degrees.

The calibration operation is shown in FIG. 2. This calibration operationmay be carried out on a test jig for example. At 200, the parametriccolor filter is assembled using the jig. As described above, this isdone using the best possible accuracy, although it should be understoodthat certain errors will inevitably occur.

In order to calibrate each filter, the filters must first becharacterized. 210 shows scanning the coated area of the filter atintervals. The scanning can use an ultraviolet or visiblespectrophotometer to scan the filter at regular intervals. The intervalsbetween scans should remain constant for each filter so that thecharacteristics of each filter are consistent from filter to filter.Each scan produces a set of data in the form of transmittance as afunction of wavelength. The scan is then analyzed to find the locationwhere the value cuts on to 50 percent. The color at any point in thefilter can then be represented as a single value.

This creates a map of points showing the optical characteristics of thefilter as a function of the position on the filter. The maps can use aspecified point in the slope curve of the data. Here, that specifiedpoint is selected to be 50 percent of the cut on value. Other slopepoints could also be selected. The map essentially becomes a table of 50percent cut on points and a position where those 50 percent cut onpoints occur. This may be stored, for example, as a 16-bit encodercount.

This produces a map at 220 indicating the position of cut on as afunction of angular position. Each map is unique to each filter.

The spectrophotometer which is used may have a maximum aperture ofscanning that may be of a different size, usually smaller than, thescanning beam used for the final illumination. For example, thespectrophotometer may have an aperture of 5 mm, while the light beam mayscan at 30 mm. Therefore, any individual scan may not be representativeof the color that would be produced when the filter is used in alighting fixture which has a much larger aperture, e.g. 30 mm. At 230,the map is compensated for the aperture mixing affect, essentiallycompensating for the larger aperture. The correction may be done bycalculating an approximate weighted mean cut on for each of a pluralityof smaller points in the map. Ideally, angular distances between scansof the filter will be an even fraction of the angle that the apertureoccupies.

An area occupied by each scan in the aperture is first calculated. Inthe system used according to the preferred mode, the coating is radiallymeasured. The aperture can be divided into radially divided segmentscentered on the center of rotation as shown in FIG. 3. The center ofrotation 300 is used as a common point. A plurality of segments areformed. Each segment has an area A1, A2 . . . An, and a correspondingmeasured wavelength λ1, for segment a1 and the like.

Once the area of each of the segments has been determined, theproportion that each segment occupies as compared with the totalaperture area is next calculated.

π r² = ∑(A₁ → A₇)${{Relative}\mspace{14mu}{area}\mspace{14mu}(a)} = \frac{{segmented}\mspace{14mu}{area}\;(A)}{\pi\; r^{2}}$

The diagram of FIG. 3 shows seven segments. Within each segment, thereis a specified 50 percent cut on represented by λ. The weighted means ofthe segments is thereforeWm=(α ₁λ₁)+(α₂λ₂)+(α₃λ₃)+(α₄λ₄)+(α₅λ₅)+(α₆λ₆)+(α₇λ₇)or

${Wm} = {\sum\limits_{i = 1}^{7}\left( {a_{t}\lambda_{1}} \right)}$

This calculation may be repeated for each practical point on the map.That is, each aperture point may be characterized fully within thecoated region of the filter.

By using this technique, most of the filter can be calibrated. Themaximum calibrated region of the filter may be 360 degrees minus theangle occupied by the clear region in the angle occupied by one wholeaperture. For example, for a 30 degree aperture and a 60 degree clearregion, 270 degrees of the filter may be calibrated.

Once this has been completed, at 240, the map or look up table isdistorted to show angular position as a function of cut on. This may bedone by interpolation. A set of target values for each filter isdetermined. These may be, for example, ideal 50 percent cut on values.These target values may be evenly spaced within the 50 percent of thecut on range of the filter's characteristic. Alternatively, they may betailored in order to increase resolution in certain areas of the filter.While both of these techniques will work, it may be essential that thesame target values be used for every like filter, in order to make surethat the calibrated values look the same from each luminaire. The numberof target values may be set to less than the number of values in themotor profiling table for a specified region.

The positions for these target values are then found by interpolation ofthe data in the distorted map. These positions are used for thecalibration process. At 250, the motor module lookup table isre-profiled using this calibration data. Each motor, such as SM1, has anassociated lookup table 131 along with servo motor drive electronics.The lookup table may include a specified number of positions, eachposition corresponding to a color. For example, there may be 49positions. These 49 positions represent the start and end points of 48line segments. These form a linear approximation to occur from which themotor moves are profiled in the 270 degree calibrated region of thefilter. The profile contains 49, 16-bit positions which extend from 8192to 57344, and are linearly spaced at one K intervals. The motorprofiling operation may move the motor to precise locations byinterpolation between points on the table.

The positions of the target values may also be in the range of8192–57344. These positions replace the linearly-spaced positions in themotor profiling table. This hence profiles the motor according to thefilter map of weighted mean 50 percent cut on values.

Using this technique allows several fixtures to be sent the same colordata by a controlling console. Each filter is moved to its uniqueposition and outputs the same color.

Although only a few embodiments have been disclosed in detail above,other modifications are possible. For example, the system above hasdescribed one specific filter. It should be understood that otherfilters, including filters on which the gradient axis is linear ortwo-dimensional could similarly be characterized. The techniques givenabove of characterizing the radial filter can be extended to linearfilters, and in many ways might be more simple in linear filters.

In addition, while this system has described distorting a lookup tablein the servo drive electronics, other ways of using this calibrationdata should also be understood. For example, the calibration data couldbe stored as the correction factor for use with existing electronics.

All such modifications are intended to be encompassed within thefollowing claims, in which:

1. An apparatus, comprising: a first unit, comprising: a first opticaldevice including a first optical filter having characteristics that varyacross a gradient axis thereof; and a first memory unit, storing firstcalibration data for the first optical filter, which first calibrationdata relates to optical characteristics which are individual to thefirst optical filter in said optical device, and which affects the waysaid first optical filter is used; a first filter moving element, whichmoves said first filter to change a position of the gradient axis thatintersects said optical axis and thereby change a characteristic offiltering, wherein said first filter moving element is responsive tosaid first calibration data; and said first filter moving elementincluding a motor, and servo electronics driving the motor, said servoelectronics including a memory table which includes a list of specifiedcolors, and positions for the specified colors, and wherein saidpositions include said first calibration data; a second unit comprising:a second optical device including a second optical filter havingcharacteristics that vary across a gradient axis thereof; and a secondmemory unit, storing second calibration data for the second opticalfilter, which second calibration data is different than said firstcalibration data and relates to optical characteristics which areindividual to the second optical filter in said optical device, andwhich affects the way said second optical filter is used; a secondfilter moving element, which moves said second filter to change aposition of the gradient axis that intersects said optical axis andthereby change a characteristic of filtering, wherein said second filtermoving element is responsive to said second calibration data, and saidsecond filter moving element including a motor, and servo electronicsdriving the motor, said servo electronics including a memory table whichincludes a list of specified colors, and positions for the specifiedcolors, and wherein said positions include said second calibration data;and a controlling console which produces color data for both of saidfirst unit and said second unit which causes both said first unit andsaid second unit to each produce one of specified color which causes thefilter moving element in the first unit to move to a first positionrepresentative of said color, and causes the filter moving element inthe second unit to go to a second position different from the firstposition, but representative of the same said color.
 2. An apparatus asin claim 1, further comprising an optical source, producing opticalenergy along an optical axis thereof, said optical axis intersectingsaid gradient axis of said optical filter.
 3. An apparatus as in claim1, wherein said filter is round and said gradient axis extends around acircumference of said filter.
 4. An apparatus as in claim 1, whereinsaid optical filter includes a position marking, marking a specifiedpoint on the optical filter.
 5. An apparatus as in claim 1, wherein saidcalibration data includes a table of points indicating a specifiedposition in a cut on curve.
 6. An apparatus as in claim 5, wherein saidspecified position is a 50 percent position.